Objectives To evaluate the association of work schedule and physical factors with fecundity.

Methods Women currently employed outside the home and trying to get pregnant (n=1739) in the Nurses’ Health Study 3 cohort (2010–2014) were included in this analysis. Work schedule and physical labour were self-reported on the baseline questionnaire, and every 6 months thereafter the women reported the duration of their ongoing pregnancy attempt. Multivariable accelerated failure time models were used to estimate time ratios (TR) and 95% CIs.

Results Among the 1739 women (median age=33 years, 93% Caucasian) the estimated proportions of women not pregnant after 12 and 24 months were 16% and 5%, respectively. None of the various shift work patterns were associated with duration of pregnancy attempt (as a surrogate for fecundity). However, women working >40 h/week had a 20% (95% CI 7 to 35%) longer median duration of pregnancy attempt compared to women working 21–40 h/week (p-trend=0.005). Women whose work entailed heavy lifting or moving (ie, 25+ pounds) >15 times/day also had a longer median duration of pregnancy attempt (adjusted TR=1.49; 95% CI 1.20 to 1.85) compared to women who never lifted or moved heavy loads (p-trend=0.002). The association between heavy moving and lifting and duration of pregnancy attempt was more pronounced among overweight or obese women (body mass index, BMI<25: TR=1.17; 95% CI 0.88 to 1.56; BMI≥25: TR=2.03, 95% CI 1.48 to 2.79; p-interaction=0.007).

Conclusions Working greater than 40 h per week and greater frequency of lifting or moving a heavy load were associated with reduced fecundity in a cohort of nurses planning pregnancy.